Mechanisms of demyelination are investigated in the following virus induced demyelinating diseases: Canine distemper; mouse hepatitis virus encephalomyelitis; murine C-type oncorna virus encephalomyelopathy and Marek's disease of chickens. The research goal is to determine whether demyelination in these conditions is due to: 1) Direct cytotoxic effect of virus on myelin supporting cells, 2) Immune reaction directed against virus laden myelin supporting cells, 3) Induction of an autoimmune reaction directed against myelin sheaths. Virus localization during the early stages of the disease prior to demyelination with particular reference to virus-plasma membrane relationship of myelin supporting cells is to be determined by electron microscopy. The pattern of demyelination occurring after viral invasion of the CNS is compared to that seen in allergic encephalomyelitis and neuritis. The effects of antibodies on virus infected cells are explored in vitro and expected to contribute to mechanisms responsible for virus persistence.